/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/DynamicMesh3.h
Copy vertex SourceVertexID from existing SourceMesh, returns new vertex id
int AppendVertex ( const [FDynamicMesh3](API\Runtime\GeometryCore\DynamicMesh\FDynamicMesh3) & SourceMesh, int SourceVertexID )